1. Field of the Invention
This invention relates to fuel cell configurations and more particularly to electrical and structural arrangements of a plurality of such solid electrolyte fuel cells.
2. Description of the Prior Art
High temperature solid electrolyte fuel cells utilizing a natural or synthetic fuel gas such as those containing hydrogen, carbon monoxide, methane, and an oxidant such as oxygen or air, are known. Such cells or stacks of cells operate above 700.degree. C. to directly convert chemical energy of the fuel into direct current electrical energy by electrochemical combustion. A typical cell, for example, reacts hydrogen fuel with oxygen from air to produce electrical energy, water vapor and heat.
Each single cell, however, provides a rather small open circuit voltage, on the order of one volt. Accordingly, plural cells are typically connected in series in order to generate a higher voltage output. Various configurations for these connections are known, such as flat plate or generally tubular arrangements. However, such interconnections have given rise to concerns which are difficult to solve. For example, such cells utilize oxide air electrodes which present high lateral ohmic losses, so that the individual cell size must be small with respect to the direction of current flow. Additionally, multiple ceramic and metallic materials are often integrated, presenting differential thermal expansion and materials compatibility concerns. To minimize such concerns, particularly the ohmic losses, the art has utilized small individual band-shaped cells, on the order of one to two centimeters in length, interconnected by good electrical conductors such as noble metals as well as non-noble metals in combination with oxide conductors. Such designs, however, do not alleviate materials compatibility concerns and require a substantial number of fabrication steps to provide an integrated electrical generator of any substantial size. Additionally, the large number of small components, layers, interconnections, and fabrication steps required in such designs raise substantial concerns regarding low-term reliability.
An example of such tubular "band" designs is provided in U.S. Pat. No. 3,525,646, which discloses a large plurality of small individual cells arranged along the circumference of a porous support tube. Each individual cell is small, and is electrically interconnected in series to the axially next cell along the tube by a conductor. A large plurality of cells, layers and conductive current collectors are required to achieve a substantial voltage. Preferably, the various layers are deposited on the support tube by plasma or flame spray methods. In addition to the concerns discussed above, such band type systems are susceptible to failure in any one cell or interconnect which defeats electrical interconnection for the entire series connected grouping of cells.
Another fuel cell arrangement is described in German Patent Disclosure No. 2,614,728, which, in an effort to obtain high power densities with the fewest possible components, forms scalloped plate type supporting bodies into a plurality of parallel channels. The channels, in conjunction with semicircular electrodes deposited therein, form fuel cells extending the length of the support bodies. The cells are interconnected through abutting radial lips and additional components. This arrangement suffers limitations similar to the deficiencies of the band designs, requiring multiple internal unaccessible connectors, and additionally depends upon the supporting plates for structural integrity. This limits the ability to electrically interconnect the cells in desireable manners, since direct contact with the cell is difficult. Additionally, the air oxidant must diffuse through the relatively thick support plates prior to gaining access to the air electrode.
It is desirable to provide a high temperature solid electrolyte fuel cell arrangement which alleviates the concerns of present designs and increases reliability.